This invention relates to a process for producing reinforced resin laminate for electrical uses such as insulating boards and printed circuit wiring boards.
The batch-type process commonly used in the manufacture of such laminates includes the steps of impregnating a porous fibrous substrate with a resin varnish in a volatile solvent, evaporating the solvent to prepare a prepreg, stacking a plurality of prepregs, and curing the stack under heat and pressure in a mold. Metal clad laminates may be prepared in a similar manner by placing a cladding metal foil on one or both sides of the stack in the mold. This batch-type process naturally requires large amount of hand labor and expensive production equipments and is, therefore, not highly productive. The use of solvent produces major safety and environmental problems.
For these and other reasons, the use of solvent-free liquid resins which are capable of curing to a solid state without producing volatile byproducts is an attractive materials for resin laminates, particularly when the manufacturing process is to be carried out in continuous mode. These liquid resins may be cured in a the absence of molding pressure and thus the difficulty of pressing a length of continuously moving green laminate may be avoided.
One serious problem which has been experienced in the production of electrical laminates from such liquid resins is the fact that fine air and other gas bubbles are unavoidably retained in the substrate and the resulting laminate includes a number of fine bubbles or voids therein. The presence of air bubbles or voids will greatly impair the performance of the finished product. For example, the thermal conductivity of the laminate would be decreased by these bubbles resulting in a decrease of heat resistance such as solder dip resistance. In case of clad laminates for use in the production of printed circuit boards, the presence of bubbles or voids weakens the bond between the insulating layer and the cladding metal foil layer and also produces inaccurate conductive patterns upon etching.
Therefore, it is an object of this invention to provide a process for producing electrical laminates, either unclad or clad, which are substantially free of bubbles or voids produced from entrapped air or other gases, from a porous fibrous substrate and a solvent-free liquid resin capable of curing to a rigid state without producing a volatile byproduct.
It is another object of the present invention to provide a process which is highly adapted to practice in the continuous mode for producing bubble-free electrical laminates having improved characteristics.
Other objects and advantages of the present invention will be apparent as the description proceeds.